Valves of the type described are frequently used in underground ground oil pipelines or the like and are controlled, manually or otherwise, by mechanisms including a rotatable nut with female threads engaging male threads on an upper part of the associated spindle. While a prolonged use of the valve may excessively damage the threads of either the spindle or the nut, the latter can be easily replaced whereas the spindle can be removed from the valve housing only together with the slider. When, e.g. after ten or more years of use, such replacement of a spindle becomes necessary, conventional practice calls for the laying of a bypass line past the valve housing and the purging of the adjoining pipe sections by a flow of nitrogen under pressure before the slider can be extracted. The defective spindle can then be detached from the slider and replaced by a new one; upon reinsertion of the slider into the valve housing, the bypass line is removed and normal operation resumes.
This procedure for the replacement of valve spindles, being generally carried out in the open. is rather expensive and time-consuming. The extraction and reinsertion of the slider requires a disassembly of the housing and a renewal of a fluidtight seal surrounding the spindle stem in the vicinity of an upper housing flange supporting the tubular attachment. Furthermore, the delivery of oil or other fluid through the line must be stopped during installation and removal of the bypass.